My best advice here is to use powers that help mess with the Borg's shields. To get their shields down quicker. Charged Particle Burst ... Boarding Party ... Target Subsystems Shields ... Tachyon Beam ... these all drain shield power. And then give you an open firing path right to the ship's hull.
Barring that you can try using exotic weaponry like Plasma (which has a special effect that will light the hull on fire directly, but it's only a small effect) ... or Antiproton weapons that have a special effect that ignore shields ... or Transphasic Torpedoes that ignore shields.
Transphasic Torpedoes are kind of special in that they were designed specifically to fight the Borg in the shows and books.
But most of those suggestions will only give you a small boost to damage. The borg are tough. I wish you luck.